In order to achieve better channel utilization and increase overall performance, multiple transmission and multiple reception antennas (also commonly referred to as multiple input, multiple output (MIMO)) at both base station (BS) and mobile station (MS) are considered.
Generally, to have better spectral efficiency, a transmitter in a MIMO system needs to know channel status information (CSI) of channels between transmitter and receiver antennas. To this end, several CSI feedback techniques have been proposed, including: i) codebook index feedback, ii) quantized channel feedback, iii) analog channel feedback, iv) use of channel reciprocity (especially for wireless communications systems operating in time division duplex (TDD) mode), and so forth.
In a wireless communications system operating in TDD mode, both downlink (DL) and uplink (UL) utilize the same frequency band, with the DL and the UL being separated by time. If the wireless communications system is using channel reciprocity, then a received signal at the BS may be used to estimate the channel from BS to a MS (i.e., the DL channel). In this case, the BS can estimate the DL channel based on the UL channel by using either an UL traffic channel or an UL sounding channel transmitted especially for UL channel estimation.
In a typical MS, there are more receiver antennas than transmitter antennas. This is due to the fact that transmitter antennas are generally more expensive to implement due to their costly power amplifiers, for example. Therefore, a BS may not have full channel status information when using channel reciprocity in a wireless communications system operating in TDD mode because each MS may transmit using only a subset of its antennas.
For example, in a wireless communications system operating in TDD mode, if a BS has K transmit antennas and K receive antennas, and a MS has N total antennas, M receive/transmit antennas, and N−M receive-only antennas with N>M, then for DL MIMO transmission, the BS is required to know the channel status information of K×N channel elements. However, since the MS transmits only on M antennas, the BS can have CSI only of K×M channel elements. Therefore, the BS does not have full CSI and the wireless communications system may not be able to operate at maximum efficiency. The BS therefore needs another type of feedback mechanism that may be used to provide channel status information for the remaining K×(N−M) channel elements.
Alternative techniques may be used to achieve full channel status information. One technique involves the use of a feedback mechanism that is used in a wireless communications system operating in frequency division duplex (FDD) mode rather than channel reciprocity. However, such feedback mechanisms usually have higher overhead than channel reciprocity.
Another technique may be to use CSI for K×M channel elements only, while leaving the remaining K×(N−M) channel elements with no CSI. However, since the BS can identify only a portion of the channel information, the transmission efficiency of the BS may be lower than what is achievable with full channel information.
Yet another technique may be to use antenna switching at the MS. With antenna switching at the MS, the MS may periodically switch it's transmit and receive antennas so that over time, the BS can have CSI for all K×N channel elements. However, this technique may require a significant amount of time to elapse before the BS can have CSI for all K×N channel elements (i.e., it may take longer to acquire the full CSI because the MS has to report CSI to the BS multiple times). Additionally, both the BS and the MS are required to know the switching pattern of the transmit and receive antennas as well as having to synchronize operations. This technique may also require the addition of switching logic at the MS, which will increase the cost of the MS.